The function body decoder stored popped arguments in a field of the
decoder class, which is a non-obvious side channel.
This CL refactors this to return the popped vector directly instead.
In local benchmarks, this has no measurable performance impact.

R=titzer@chromium.org

Bug: v8:9183
Change-Id: I79b28e349e3009b9f01be25f116c4085ca569ca7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1596732Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61379}